Hovering flight in hummingbird hawkmoths: Kinematics, wake dynamics and aerodynamic power

Research output: Contribution to journalArticle

Abstract

Hovering insects are divided into two categories: ‘normal’ hoverers that move the wing symmetrically in a horizontal stroke plane, and those with an inclined stroke plane. Normal hoverers have been suggested to support their weight during both downstroke and upstroke, shedding vortex rings each half-stroke. Insects with an inclined stroke plane should, according to theory, produce flight forces only during downstroke, and only generate one set of vortices. The type of hovering is thus linked to the power required to hover. Previous efforts to characterize the wake of hovering insects have used low-resolution experimental techniques or simulated the flow using computational fluid dynamics, and so it remains to be determined whether insect wakes can be represented by any of the suggested models. Here, we used tomographic particle image velocimetry, with a horizontal measurement volume placed below the animals, to show that the wake shed by hovering hawkmoths is best described as a series of bilateral, stacked vortex ‘rings’. While the upstroke is aerodynamically active, despite an inclined stroke plane, it produces weaker vortices than the downstroke. In addition, compared with the near wake, the far wake lacks structure and is less concentrated. Both near and far wakes are clearly affected by vortex interactions, suggesting caution is required when interpreting wake topologies. We also estimated induced power (Pind) from downwash velocities in the wake. Standard models predicted a Pind more than double that from our wake measurements. Our results thus question some model assumptions and we propose a reevaluation of the model parameters.

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Authors
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Research areas and keywords

Subject classification (UKÄ) – MANDATORY

  • Zoology
  • Evolutionary Biology

Keywords

  • Aerodynamics, Flight model, Hovering, Macroglossum, Particle image velocimetry, Vortex wake
Original languageEnglish
Article numberjeb230920
JournalJournal of Experimental Biology
Volume224
Issue number10
Publication statusPublished - 2021 May
Publication categoryResearch
Peer-reviewedYes

Bibliographic note

Funding Information: The tomo-PIV system was funded by an infrastructure grant from Lund University to A.H., and the research was funded by the Swedish Research Council (Vetenskapsrådet; 621-2012-3585, 2016-03625 to A.H. and 621-2013-4596, 2017-03890 to L.C.J.). The Knut and Alice Wallenberg Foundation provided funds to obtain the kinematics cameras. This is a report from the Centre for Animal Movement Research (CAnMove) funded by a Linnaeus grant from the Swedish Research Council (349-2007-8690) and Lund University. Open Access funding was provided by Lund University. Deposited in PMC for immediate release. Funding Information: The tomo-PIV system was funded by an infrastructure grant from Lund University to A.H., and the research was funded by the Swedish Research Council (Vetenskapsr?det; 621-2012-3585, 2016-03625 to A.H. and 621-2013-4596, 2017-03890 to L.C.J.). The Knut and Alice Wallenberg Foundation provided funds to obtain the kinematics cameras. This is a report from the Centre for Animal Movement Research (CAnMove) funded by a Linnaeus grant from the Swedish Research Council (349-2007-8690) and Lund University. Open Access funding was provided by Lund University. Deposited in PMC for immediate release. Publisher Copyright: © 2021. Published by The Company of Biologists Ltd. Copyright: Copyright 2021 Elsevier B.V., All rights reserved.